1. Field of the Invention
The present invention relates to a linear image sensor, an image reading apparatus, an image reading method, and a program, and in particular to a linear image sensor capable of reading an image at different degrees of image resolution, an image reading apparatus using the linear image sensor, an image reading method applied to the image reading apparatus, and a program for causing a computer to implement the image reading method.
2. Description of the Related Art
In general, in an image reading apparatus such as an image scanner, a light receiving unit that reads an image, and converts the read image into an electric signal is comprised of a linear image sensor formed of photo sensors (photoelectric conversion elements) arranged in an array. Such a light receiving unit has pixels corresponding in number to the size of an original to be subjected to image reading, as well as to the sampling density. Further, the light receiving unit is generally comprised of a plurality of sensors with color filters corresponding to red, green and blue colors, respectively.
Conventionally, there has been known an image reading apparatus which reads an image at a resolution of not more than 2400, for example, dpi by receiving light reflected from an original. When it is desired to read a transparent original, such as a photo film, at a resolution of e.g. 4800 dpi, it is necessary to increase the sensor element density of the light receiving unit. However, there is a limit to increase of the sensing element density within a limited space.
To address the problem, U.S. Pat. No. 6,496,286 has proposed an image reading apparatus which employs two rows of linear image sensors having the same image resolution and arranged at the same pitch, one of the sensor arrays being shifted from the other by half the pitch. Image signals are read alternately from sensor elements of one of the two linear image sensors and sensor elements of the other in a zigzag manner, and a spatial positional difference between the two linear image sensors is compensated to double the above resolution.
Further, Japanese Laid-Open Patent Publication (Kokai) No. H10-327294 has proposed an image reading apparatus which has a linear image sensor with an enhanced resolution in addition to a linear image sensor with a normal resolution, and in which these linear image sensors are selectively used to carry out image reading at different resolutions.
However, in general, cases where reading of transparent originals, such as a photo film, which generally requires enhanced resolution reading occur at a lower frequency than cases where reading of originals at a normal resolution. Nonetheless, if enhanced resolution reading is always carried out, image reading at a normal resolution involves unnecessary image processing, taking an unnecessarily long time for the processing to be carried out.
Moreover, for the image reading at an enhanced resolution, which takes place only at low frequency, an expensive linear image sensor with a high resolution is required, increasing the cost of the image reading apparatus.